1. Technical Field
The present invention relates generally to vibrators, transducers, and associated apparatus, and more specifically to an improved method and apparatus for generating a wide bandwidth vibrational stimulus to the body of a user in response to an electrical input.
2. Description of Related Art
The sense of feel is not typically used as a man-machine communication channel. However, it is as acute and in some instances as important as the senses of sight and sound. Tactile stimuli provide a silent and invisible, yet reliable and easily interpreted, communication channel using the human sense of touch. Information can be transferred in various ways including force, pressure, and frequency-dependent mechanical stimulus. Broadly, this field is also known as haptics.
Haptic interfaces may be employed to provide additional sensory feedback during interactive tasks. For example computer games make use of portable game consoles that often include various motors and transducers that apply forces to the housing of the console at various vibrational rates and levels. These forces correlate to actions or activities within the game and improve the gaming experience. Similar haptic interface techniques may be employed for a variety of other interface tasks including vibrotactile communication via a flat panel touch screens or mobile device. Vibration feedback may be more intuitive than audio feedback and has been shown to improve user performance with certain devices.
A single vibrotactile transducer may be employed for a simple purpose such as sending an alert, e.g., via a mobile phone. Many interface devices, such as computer interface devices, allow some form of haptic feedback to the user. A plurality of vibrotactile transducers may be employed to provide more detailed information, such as spatial orientation of a person relative to some external reference. Using an intuitive organization of vibrotactile stimuli, information referenced with respect to a user's body (body-referenced) may be communicated to a user. Such vibrotactile displays have been shown to reduce perceived workload by its ease in interpretation and intuitive nature.
Vibrotactile transducers may be wearable, mounted within the padding of a seat back and/or base, or included within the structure of an interface device, such as a PDA or gaming interface. In each case, the vibrotactile transducer preferably provides a sufficiently strong, localized vibrotactile sensation (stimulus) to the body. These devices should preferably be small, lightweight, efficient, electrically and mechanically safe and reliable in harsh environments. Moreover, drive circuitry should be compatible with standard communication protocols to allow simple interfacing with various avionics and other systems.
The study and development of mechanical and/or vibrational stimuli on the human skin has been ongoing. For example, a particular diagnostic device produces and monitors mechanical stimulation against the skin using a moving mass contactor termed a “tappet” (plunger mechanical stimulator). A bearing and shaft links and guides the tappet to the skin, and an electromagnetic motor circuit provides linear drive, similar to that used in a moving-coil loudspeaker. The housing of the device is large and mounted to a rigid stand and support, and only the tappet makes contact with the skin. The reaction force from the motion of the tappet is applied to a massive object such as the housing and the mounting arrangement. The device was developed for laboratory experiments and is not intended to provide information to a user by means of vibrational stimuli or to be implemented as a wearable device.
Various other types of vibrotactile transducers that provide a tactile stimulus to the body of a user have been produced. Other vibrotactile transducers designs have incorporated electromagnetic devices based on a voice coil (loudspeaker or shaker) design, an electrical solenoid design, or a simple variable reluctance design. The most common approach is the use of a small motor with an eccentric mass rotating on the shaft, such as is used in pagers and cellular phones. A common shortcoming of these previous design approaches is that the transducers are rapidly damped when operated against the body, usually due to the mass loading of the skin or the transducer mounting arrangement.
Eccentric mass (EM) motors, e.g., pager motors, are usually constructed with a DC motor with an eccentric mass load, such as half-circular cylinder that is mounted onto the motor's shaft. The motor is designed to rotate the shaft and its off-center (eccentric) mass load at various speeds. From the conservation of angular momentum, the eccentric mass imparts momentum to the motor shaft and consequently the motor housing. The angular momentum imparted to the motor housing depends on the mounting of the motor housing, the total mass of the motor, the mass of the eccentric rotating mass, the radius of the center of mass from the shaft, and the rotational velocity. In steady state, the angular momentum imparted to the housing results in three dimensional motion and a complex orbit that depends on the length of the motor, the mounting geometry, the length of the shaft, and center of gravity of the moving masses. This implementation applies forces in a continually changing direction, confined to a plane of rotation of the mass. Thus, the resultant motion of the motor housing is three dimensional and complex. If this motion is translated to an adjacent body, the complex vibration (and perceived vibrational stimulus) may be interpreted to be a diffuse “wobble” sensation.
The rpm of the EM motor defines the tactile frequency stimulus and is typically in the range of 60-150 Hz. These devices are generally intended to operate at a single (relatively low) frequency, and cannot be optimized for operating over a wide frequency range or at sufficiently high frequencies where the skin of the human body is most sensitive to vibrational stimuli. It may be possible to increase the vibrational frequency on some FM motors by increasing the speed of the motor (for example, by increasing the applied voltage to a DC motor). However, there are practical limits to this approach, as the force imparted to the bearing increases with rotational velocity and the motor windings are designed to support a maximum current. The angular momentum and therefore the eccentric motor vibrational output (and force) also increase with rotational velocity which limits use of the device over bandwidth. In fact, in some designs, the force and rotational rate are coupled and cannot be separated.
The temporal resolution of EM motors is limited by the start up (spin-up) times which can be relatively long, e.g., on the order of about 100 ms. This is somewhat longer than the temporal resolution by the skin, and thus, can limit data rates. If the vibrotactile feedback is combined with other sensory feedback such as visual or audio, the start-up delay has the potential of introducing disorientation. The slow response time needed to achieve a desired rotational velocity is due the acceleration and deceleration of the spinning mass. Some motor control methods can address this problem by increasing the initial torque when initially turned on. Motors with smaller eccentric masses may be easier to drive (and reduce spin-up time), but thus far a reduced eccentric mass also results in an actuator that produces a lower vibrational amplitude.
There are two important effects associated with the practical operation of EM motors as vibrotacile or other transducers. Firstly, the motion that is translated to an adjacent body depends on the loading on the motor housing. From the conservation of momentum, the greater the mass loading on the motor (or transducer housing) the lower the vibrational velocity and perceived amplitude stimulus. Secondly, from the conservation of momentum, if the mass loading on the motor is changed, the torque on the motor and angular rotation rate also changes. This may be undesirable from a control standpoint, and in the limiting case, a highly loaded transducer may produce minimal displacement output and thus be ineffective as a tactile stimulus. In fact, there have been several reports of inconsistency in results which may be attributed to the shortcomings of other designs and modeling attempts to overcome this using complex mounting.
In one system, a computer mouse haptic interface and transducer uses a motor transducer. A mechanical flexure system converts rotary force from the motor to allow a portion of the housing flexure to be linearly moved. This approach relies on a complex mechanical linkage that is both expensive to implement and at high rotational velocities prone to deleterious effects of friction. It is therefore only suited to very low frequency haptic feedback.
In another system, a mechanically movable eccentric mass is employed in an effort to control the start-up and force characteristics of an eccentric mass motor. However, this approach is mechanically complex and not intended to be wide band.
In yet another system, an EM motor is connected to the housing via a compliant spring. The system makes up a two degree of freedom resonant mechanical system. The motor mass and spring systems are completely contained within a rigid housing. The movement of the motor mass in this case acts to impart an inertial force to the housing. This type of transducer configuration is known as a “shaker.” The design claims improved efficiency and the ability to be driven by a harmonic motor drive for use as a haptic force feedback computer interface. The system does not address any loading on the housing and in fact assumes that there are no other masses or mechanical impedances acting on the exterior of the housing. Further, this design is narrow band thereby limiting the effectiveness and use of this approach.
Employing linear “shaker” transducers, another system employs a low frequency vibrator with a reciprocating piston mass within a low friction bearing, actuated by an electromagnetic with a magnetic spring, having a spring constant K. The ratio of K to the mass M of the reciprocating member is made to be resonant in the operating frequency range of the vibrator.